Piezoelectric bulk layers with tilted c-axis orientation and methods for making the same

ABSTRACT

Bulk acoustic wave resonator structures include a bulk layer with inclined c-axis hexagonal crystal structure piezoelectric material supported by a substrate. The bulk layer may be prepared without first depositing a seed layer on the substrate. The bulk material layer has a c-axis tilt of about 32 degrees or greater. The bulk material layer may exhibit a ratio of shear coupling to longitudinal coupling of 1.25 or greater during excitation. A method for preparing a crystalline bulk layer having a c-axis tilt includes depositing a bulk material layer directly onto a substrate at an off-normal incidence. The deposition conditions may include a pressure of less than 5 mTorr and a deposition angle of about 35 degrees to about 85 degrees.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/646,212, filed 21 Mar. 2018, the disclosure of which is incorporatedby reference herein in its entirety.

FIELD

The present disclosure relates to structures including inclined c-axishexagonal crystal structure piezoelectric materials such as aluminumnitride (AlN) and zinc oxide (ZnO), as well as systems and methods forproducing inclined c-axis hexagonal crystal structure piezoelectricmaterial. Inclined c-axis hexagonal crystal structure piezoelectricmaterials may be used, for example, in various resonators as well as inthin film electroacoustic and/or sensor devices, particularly forsensors operating in liquid/viscous media (e.g., chemical andbiochemical sensors), and the like.

BACKGROUND

Hexagonal crystal structure piezoelectric materials such as AlN and ZnOare of commercial interest due to their piezoelectric andelectroacoustic properties. Beneficial properties in this regard includea high mechanical quality factor, moderate coupling coefficient,moderate piezoelectric constant, high acoustic velocity, and lowpropagation losses. In addition to these characteristics, AlN thin filmsare chemically stable and compatible with various integrated circuitfabrication technologies, thereby making AlN an attractive material forfabrication of electroacoustic devices, including bulk acoustic wave(BAW) devices such as bandpass filters and the like.

A primary use of electroacoustic technology has been in thetelecommunication field (e.g., for oscillators, filters, delay lines,etc.). More recently, there has been a growing interest in usingelectroacoustic devices in high frequency sensing applications due tothe potential for high sensitivity, resolution, and reliability.However, it is not trivial to apply electroacoustic technology incertain sensor applications—particularly sensors operating in liquid orviscous media (e.g., chemical and biochemical sensors)—sincelongitudinal and surface waves exhibit considerable acoustic leakageinto such media, thereby resulting in reduced resolution.

In the case of a piezoelectric crystal resonator, an acoustic wave mayembody either a bulk acoustic wave (BAW) propagating through theinterior (or ‘bulk’) of a piezoelectric material, or a surface acousticwave (SAW) propagating on the surface of the piezoelectric material. SAWdevices involve transduction of acoustic waves (commonly includingtwo-dimensional Rayleigh waves) utilizing interdigital transducers alongthe surface of a piezoelectric material, with the waves being confinedto a penetration depth of about one wavelength. BAW devices typicallyinvolve transduction of an acoustic wave using electrodes arranged onopposing top and bottom surfaces of a piezoelectric material. In a BAWdevice, different vibration modes can propagate in the bulk material,including a longitudinal mode and two differently polarized shear modes,wherein the longitudinal and shear bulk modes propagate at differentvelocities. The longitudinal mode is characterized by compression andelongation in the direction of the propagation, whereas the shear modesconsist of motion perpendicular to the direction of propagation with nolocal change of volume. The propagation characteristics of these bulkmodes depend on the material properties and propagation directionrespective to the crystal axis orientations. Because shear waves exhibita very low penetration depth into a liquid, a device with pure orpredominant shear modes can operate in liquids without significantradiation losses (in contrast with longitudinal waves, which can beradiated in liquid and exhibit significant propagation losses).Restated, shear mode vibrations are beneficial for operation of acousticwave devices with fluids because shear waves do not impart significantenergy into fluids.

Certain piezoelectric thin films are capable of exciting bothlongitudinal and shear mode resonance. To excite a wave including ashear mode using a standard sandwiched electrode configuration device, apolarization axis in a piezoelectric thin film must generally benon-perpendicular to (e.g., tilted relative to) the film plane.Hexagonal crystal structure piezoelectric materials such as (but notlimited to) aluminum nitride (AlN) and zinc oxide (ZnO) tend to developtheir polarization axis (i.e., c-axis) perpendicular to the film plane,since the (0001) plane typically has the lowest surface density and isthermodynamically preferred. Certain high-temperature (e.g., vapor phaseepitaxy) processes may be used to grow tilted c-axis films, butproviding full compatibility with microelectronic structures such asmetal electrodes or traces requires a low temperature deposition process(e.g., typically below about 300° C.).

Low temperature deposition methods such as reactive radio frequencymagnetron sputtering have been used for preparing tilted AlN filmshaving angles that vary significantly with position over the area of asubstrate. FIG. 1 is a simplified schematic of an axial sputteringdeposition apparatus arranged to eject metal atoms from a target 2(adjacent to a cathode (not shown)) toward a substrate 4 supported by asubstrate holder 6 that is substantially parallel to the target 2 in areactive gas-containing environment. The target 2 and the substrate 4are aligned with one another and share a single central axis 8; however,a typical geometry of sputtering deposition results in a cosinedistribution 10 of piezoelectric material molecules (e.g., AlN moleculescreated by metal atoms reacting with nitrogen in the sputtering gas)being received by the substrate 4. This phenomenon leads to a c-axisdirection of the deposited piezoelectric material that varies withradial position, from an angle of zero (corresponding to a verticalc-axis) at the center of the substrate 4, to a c-axis direction with atilt angle that increases with distance from the center.

The above-described variation with radial position of c-axis directionof a deposited piezoelectric material is disclosed by Stan, G. E., etal., “Tilt c Axis Crystallite Growth of Aluminium Nitride Films byReactive RF-Magnetron Sputtering,” Digest Journal of Nanomaterials andBiostructures, vol. 7, no. 1, pp. 41-50 (2012) (hereinafter, “Stan”).FIG. 2A is a schematic representation (reproduced from Stan) of arocking curve measurement geometry of an AlN film structure obtained byradio frequency magnetron sputtering in a reactive gas environment in anaxially aligned planar sputtering system without tilting a 50 mm Sisubstrate. FIG. 2A shows that the AlN film structure according to Stanexhibits zero c-axis tilt angle at the center, and a radiallysymmetrical variation of tilt angle of crystallites in the AlN filmstructure, analogous to a circular “race track” with banked walls. FIG.2B is a plot of tilt angle versus distance from center (also derivedfrom Stan) for the AlN film structure described in connection with FIG.2A, showing a nearly linear variation of tilt angle with increasingdistance away from a center of the AlN film structure, to a maximum tiltangle of about 6.5 degrees near the margins of the 50 mm wafer. Oneeffect of the lack of uniformity of c-axis tilt angle of the AlN filmstructure over the substrate is that if the AlN film-covered substratewere to be diced into individual chips, the individual chips wouldexhibit significant variation in c-axis tilt angle and concomitantvariation in acoustic wave propagation characteristics. Such variationin c-axis tilt angle would render it difficult to efficiently producelarge numbers of resonator chips with consistent and repeatableperformance. Moreover, use of a target surface axially aligned with asubstrate holder that is parallel to the target surface enablesattainment of only a limited range of c-axis tilt angles, as evidencedby the 0-6.5 degree tilt angle range shown in FIG. 2B.

Before describing other techniques for preparing tilted AlN films,desirable regimes for c-axis tilt angle (or angle of inclination) willbe discussed. An electromechanical coupling coefficient is a numericalvalue that represents the efficiency of piezoelectric materials inconverting electrical energy into acoustic energy for a given acousticmode. Changing the c-axis angle of inclination for hexagonal crystalstructure piezoelectric materials causes variation in shear andlongitudinal coupling coefficients, as shown in FIG. 3. FIG. 3 embodiesplots of shear coupling coefficient (K_(s)) and longitudinal couplingcoefficient (K_(l)) each as a function of c-axis angle of inclinationfor AlN. It can be seen that a maximum electromechanical couplingcoefficient of shear mode resonance in AlN is obtained at a c-axis angleof inclination of about 35 degrees, that a pure shear response (withzero longitudinal coupling) is obtained at a c-axis angle of inclinationof about 46 degrees, and that the shear coupling coefficient exceeds thelongitudinal coupling coefficient for c-axis angle of inclination valuesin a range of from about 19 degrees to about 63 degrees. Thelongitudinal coupling coefficient is also zero at a c-axis angle ofinclination of 90 degrees, but it is impractical to grow AlN at verysteep c-axis inclination angles. Similar behavior is expected for otherpiezoelectric materials, although the specific angle positions may vary.For electroacoustic resonators intended to operate in liquids or otherviscous media, it would be desirable to provide piezoelectric films witha c-axis tilt angle sufficient to provide a shear coupling coefficientthat exceeds a longitudinal coupling coefficient—in certain embodiments,at a c-axis tilt angle in which the longitudinal coupling coefficientapproaches zero, or at a c-axis-tilt angle at or near a value whereshear coupling is maximized. Thus, for an electroacoustic resonatorincluding an AlN piezoelectric layer, it would be desirable to provide ac-axis tilt angle in a range of from about 19 degrees to about 63degrees, and particularly desirable to provide a c-axis tilt anglebetween 35 and 46 degrees. Other c-axis tilt angles may be desirable forother purposes or when materials other than AlN are used for deposition.

Various low temperature deposition methods that have been devised forgrowing AlN films at c-axis tilt angles greater than those attainablewith the axial sputtering apparatus of FIG. 1 are described inconnection with FIGS. 4A-4C. FIG. 4A (which is adapted from Moreira,Milena De Albuquerque, “Synthesis of Thin Piezoelectric AlN Films inView of Sensors and Telecom Applications,” 2014, Digital ComprehensiveSummaries of Uppsala Dissertations from the Faculty of Science andTechnology, 1651-6214; 1160) (hereinafter, “Moreira”) is a simplifiedschematic of an off-axis sputtering deposition apparatus arranged toeject metal atoms from a target surface 12 toward a substrate 14supported by a substrate holder 16 that is substantially parallel to thetarget surface 12, with central axes of the substrate 14 and the targetsurface 12 being parallel but offset relative to one another, with anangle θ representing an angle between a central axis 18 of the targetsurface 12 and a center of the substrate 14. A distribution 20 ofpiezoelectric material molecules created by reaction of metal atoms andgas is received by the substrate 14, resulting in a tilted c-axisdirection of deposited piezoelectric material (including tilted‘columns’ of piezoelectric material with a preferential growth directiondue to alignment with the tilted flux), with the c-axis tilt angle ofthe piezoelectric material varying with respect to position along thesubstrate 14. In particular, a portion of the deposited piezoelectricmaterial that is closer to the central axis 18 of the target surface 12will exhibit a c-axis tilt angle that is shallower than a portion of thepiezoelectric material that is farther from the central axis 18 of thetarget surface 12. Due to the lateral offset of the substrate 14relative to the central axis 18 of the target surface 12, the off-axissputtering deposition apparatus of FIG. 4A is capable of attainingpiezoelectric films with c-axis tilt angles that are larger than thoseattainable with the apparatus of FIG. 1.

Additional low temperature deposition sputtering apparatuses capable ofgrowing piezoelectric films with even larger c-axis tilt angles aredescribed in connection with FIGS. 4B and 4C (which are also adaptedfrom Moreira). FIG. 4B is a simplified schematic of a sputteringdeposition apparatus arranged to eject metal atoms from a target surface22 toward a substrate 24 supported by a substrate holder 26 that isnon-parallel to the target surface 22 (i.e., wherein the substrateholder 26 is tilted by an angle θ relative to a plane parallel with thetarget surface 22), wherein a central axis 28 of the target surface 22extends through a center point of the substrate 24. A distribution 30 ofpiezoelectric material molecules created by reaction of metal atoms andgas is received by the substrate 24, resulting in a tilted c-axisdirection of deposited piezoelectric material (including tilted‘columns’ of piezoelectric material with a preferential growth directiondue to alignment with the tilted flux), with the c-axis tilt angle ofthe piezoelectric material varying with respect to position along thesubstrate 24. FIG. 4C is a simplified schematic of a sputteringdeposition apparatus arranged to eject metal atoms from a target surface32 toward a substrate 34 supported by a substrate holder 36 that isnon-parallel to the target surface 32. A central axis 38B of thesubstrate 34 extends through a center point of the target surface 32,with a central axis 38A of the target surface 32 being separated fromthe central axis 38B of the substrate 34 by a first angle θ₁, and withsubstrate holder 36 being tilted by a second angle θ₂ relative to aplane parallel with the target surface 32. A distribution 40 ofpiezoelectric material molecules created by reaction of metal atoms andgas is received by the substrate 34, resulting in a tilted c-axisdirection of deposited piezoelectric material (including tilted‘columns’ of piezoelectric material with a preferential growth directiondue to alignment with the tilted flux), with the c-axis tilt angle ofthe piezoelectric material varying with respect to position along thesubstrate 34.

Yet another method for growing a tilted c-axis AlN film involvestwo-step sputtering deposition as described by Moreira, including firststep growth of an initial, non-textured seed layer at a relatively highprocess pressure while keeping the substrate at room temperature,followed by second step growth of a film at a lower process pressure andan elevated substrate temperature. FIG. 5A is a cross-sectionalschematic view of a seed layer 44 exhibiting multiple textures depositedvia the first sputtering step over a substrate 42, and FIG. 5B is asimilar view of the seed layer 44 and substrate 42 of FIG. 5A followingdeposition via a second sputtering step of a tilted axis AlN film 46bulk layer over the seed layer 44. As described by Moreira, the seedlayer exhibits different textures, most notably (103) and (002). Thehigh pressure used during seed layer deposition promotes the (103)texture, and typically results in a seed layer with no in-planealignment along the (002) direction.

When a bulk layer is deposited, the pressure is decreased to actuallypromote the evolution of (002) texture. The seed layer grains with (002)orientations facing the incoming flux during an off-normal incidencedeposition will provide crystallographic templates such that theresultant bulk layer will have tilted (002) texture. The low pressuredeposition in combination with a small distance between the target andsubstrate yields a directional deposition flux that results incompetitive column growth. During off-normal incidence deposition, (002)grains grow taller than other grains due to shadowing effects andsurface atom mobility differences. Adatoms on (002) grains are slowerthan those on other grains (e.g., on non-tilted (002) grains, or tiltedor non-tilted (103) grains). Because the mobilities of adatoms on (002)grains are slower, they have higher “sticking coefficients” than thefaster adatoms on other grains, which results in a higher verticalgrowth of the (002) grains. As (002) grains get taller they shadowneighboring grains due to incoming flux coming from an angle, therebyfurther limiting growth of such neighboring grains. Competitive columngrowth results in a film with a c-axis lying in the plane of thedeposition flux at any given point along the substrate. As noted byMoreira, even though there is no intentional tilt of the flux, themagnetron disposition at the target surface generates a “race track”,which in turn provides the tilted flux direction towards the substrate.Such a “race track” described by Moreira is understood to correspond toa radially symmetric variation of tilt angle of crystallites in the filmstructure, similar to that described hereinabove in connection withFIGS. 2A and 2B.

Each of the foregoing apparatuses and tilted piezoelectric materialgrowth methods are understood to produce film-covered substratesexhibiting significant variation in c-axis tilt angle with respect toposition on the substrate. As noted previously, one effect of a lack ofuniformity of c-axis tilt angle of a piezoelectric film arranged on thesubstrate is that if the film-covered substrate were to be diced intoindividual chips, then the individual chips would exhibit significantvariation in c-axis tilt angle and concomitant variation in acousticwave propagation characteristics. Such variation in c-axis tilt anglewould render it difficult to efficiently produce large numbers ofresonator chips with consistent and repeatable performance.

Improved methods and systems for producing bulk films with c-axis tilthave been described, where the c-axis tilt of the bulk layer isprimarily controlled by controlling the deposition angle. For example, adevice and method for depositing seed and bulk layers with a tiltedc-axis are described in U.S. patent application Ser. No. 15/293,063entitled “Deposition System for Growth of Inclined C-Axis PiezoelectricMaterial Structures;” U.S. patent application Ser. No. 15/293,071entitled “Methods for Fabricating Acoustic Structure with InclinedC-Axis Piezoelectric Bulk and Crystalline Seed Layers;” U.S. patentapplication Ser. No. 15/293,082 entitled “Acoustic Resonator Structurewith Inclined C-Axis Piezoelectric Bulk and Crystalline Seed Layers;”U.S. patent application Ser. No. 15/293,091 entitled “Multi-StageDeposition System for Growth of Inclined C-Axis Piezoelectric MaterialStructures;” and U.S. patent application Ser. No. 15/293,108 entitled“Methods for Producing Piezoelectric Bulk and Crystalline Seed Layers ofDifferent C-Axis Orientation Distributions”.

These patent publications also describe attempts to deposit a bulk layerdirectly onto a substrate without first depositing a seed layer (see,for example, U.S. patent application Ser. No. 15/293,071). However, suchbulk layers, deposited under the same conditions as used for bulk layersdeposited onto seed layers, failed to exhibit a desired minimum shearmode to longitudinal coupling ratio of at least 1.25. In other words,the resulting structures would not be useful for bulk acoustic sensingresonators in liquid/viscous media, which was one of the intendedfunctions of these published patent applications. When the bulk layerwas deposited onto a seed layer that was deposited at 5 mTorr, theresulting films exhibited insufficient shear mode coupling. When thebulk layer was deposited onto a seed layer that was deposited at 15mTorr, the resulting films exceeded the desired minimum ratio of 1.25and thus would be useful for bulk acoustic resonators in liquid/viscousenvironments.

Further improvements are desired to provide, for example, one or moreof: additional control over the angle of the c-axis of crystals in bulklayers; improved characteristics such as mechanical quality factor,coupling coefficient, or shear to longitudinal coupling ratio; andimproved manufacturing efficiency of bulk layers.

SUMMARY

The present disclosure provides bulk acoustic wave resonator structuresand methods for fabricating such resonator structures. The bulk acousticwave resonator structures include a bulk layer with inclined c-axishexagonal crystal structure piezoelectric material. The hexagonalcrystal structure piezoelectric material bulk layer is supported by asubstrate. The bulk layer may be prepared without first depositing aseed layer on the substrate. The bulk layer is prepared such that thec-axis orientation of the crystals in the bulk layer is selectablewithin a range of about 0 degrees to about 90 degrees, such as fromabout 30 degrees to about 52 degrees, or from about 35 degrees to about46 degrees. The c-axis orientation distribution is preferablysubstantially uniform over the area of a large substrate (e.g., having adiameter in a range of at least about 50 mm, about 100 mm, or about 150mm), thereby enabling multiple chips to be derived from a singlesubstrate and having the same or similar acoustic wave propagationcharacteristics.

A method for preparing a crystalline bulk layer having a c-axis tiltincludes depositing a bulk material layer directly onto a substrate atan off-normal incidence under deposition conditions comprising apressure of less than 5 mTorr. The bulk material layer may be depositedat a deposition angle of about 35 degrees to about 85 degrees.

A method for preparing a crystalline bulk layer having a c-axis tiltincludes depositing a bulk material layer directly onto a substrate atan off-normal incidence, where the bulk material layer has a thicknessof about 1,000 Angstroms to about 30,000 Angstroms. The bulk materiallayer may be deposited at a deposition angle of about 35 degrees toabout 85 degrees. The bulk material may exhibit a ratio of shearcoupling to longitudinal coupling of 1.25 or greater during excitation.

A structure includes a substrate comprising a wafer and a piezoelectricbulk material layer deposited onto a surface of the wafer, where thebulk material layer has a c-axis tilt of about 32 degrees or greater.The structure may exhibit a ratio of shear coupling to longitudinalcoupling of 1.25 or greater during excitation.

A bulk acoustic wave resonator includes a structure including asubstrate comprising a wafer and a piezoelectric bulk material layerdeposited onto a surface of the wafer, where the bulk material layer hasa c-axis tilt of about 32 degrees or greater, where at least a portionof piezoelectric bulk material layer is between the first electrode andthe second electrode.

A method for preparing a crystalline bulk layer having a c-axis tiltwith a preselected angle includes depositing a bulk material layer ontoa substrate at an off-normal incidence under initial conditions thatretard surface mobility of the material being deposited such thatcrystals in the bulk material layer are substantially parallel to oneanother and are substantially oriented in a direction of the preselectedangle.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a simplified schematic of an axial sputtering depositionapparatus arranged to eject metal atoms from a target surface toward asubstrate supported by a substrate holder substantially parallel to thetarget surface, with central axes of the target surface and thesubstrate being aligned with one another.

FIG. 2A is a schematic representation of a rocking curve measurementgeometry of an AlN film structure obtained by radio frequency magnetronsputtering in a reactive gas environment in a planar sputtering systemwithout tilting the substrate, showing zero tilt angle at the center,and a radially symmetrical variation of tilt angle of crystallites inthe AlN film structure.

FIG. 2B is a plot of tilt angle versus distance from center for the AlNfilm structure described in connection with FIG. 2A, showing a nearlylinear variation of tilt angle with increasing distance away from acenter of the AlN film structure.

FIG. 3 is a plot of shear coupling coefficient (K_(s)) and longitudinalcoupling coefficient (K_(l)) as a function of c-axis angle ofinclination for AlN.

FIG. 4A is a simplified schematic of an off-axis sputtering depositionapparatus arranged to eject metal atoms from a target surface toward asubstrate supported by a substrate holder substantially parallel to thetarget surface, with central axes of the substrate and the targetsurface being parallel but offset relative to one another.

FIG. 4B is a simplified schematic of a sputtering deposition apparatusarranged to eject metal atoms from a target surface toward a substratesupported by a substrate holder that is non-parallel to the targetsurface, with a central axis of the target surface extending through acenter point of the substrate.

FIG. 4C is a simplified schematic of a sputtering deposition apparatusarranged to eject metal atoms from a target surface toward a substratesupported by a substrate holder that is non-parallel to the targetsurface, with a central axis of the substrate extending through a centerpoint of the target surface, and with a central axis of the targetsurface being separated from the central axis of the substrate by afirst angle, and with the substrate holder being tilted by a secondangle relative to a plane parallel with the target surface.

FIG. 5A is a cross-sectional schematic view of a seed layer exhibitingmultiple textures (e.g., (103) and (002)) deposited via a firstsputtering step over a substrate.

FIG. 5B is a cross-sectional schematic view of the seed layer andsubstrate of FIG. 5A following deposition via a second sputtering stepof a tilted axis AlN film over the seed layer.

FIG. 6A is a graphical representation of the c-axis orientationdistribution resulting from a prior art method without a seed layer.

FIG. 6B is a graphical representation of the c-axis orientationdistribution resulting from a prior art method with a seed layerprepared at a pressure of 5 mTorr.

FIG. 6C is a graphical representation of the c-axis orientationdistribution resulting from a prior art method with a seed layerprepared at a pressure of 15 mTorr.

FIG. 7 is an upper exterior perspective view of a reactor of adeposition system for growing a hexagonal crystal structurepiezoelectric material with an inclined c-axis, the system including alinear sputtering apparatus, a movable substrate table for supportingmultiple substrates, and a collimator.

FIG. 8 is an upper perspective view of some of the elements of thereactor of FIG. 7, including a linear sputtering apparatus, atranslation track for translating a movable substrate table forsupporting multiple substrates, and a collimator.

FIG. 9 is a schematic cross-sectional view of a portion of a bulkacoustic wave solidly mounted resonator device including an inclinedc-axis hexagonal crystal structure piezoelectric material bulk layer asdisclosed herein, with the resonator device including an active regionwith a portion of the piezoelectric material arranged betweenoverlapping portions of a top side electrode and a bottom sideelectrode.

FIG. 10 is a schematic cross-sectional view of a film bulk acoustic waveresonator (FBAR) device according to one embodiment including aninclined c-axis hexagonal crystal structure piezoelectric material bulklayer arranged over a crystalline seed layer as disclosed herein, withthe FBAR device including a substrate defining a cavity covered by asupport layer, and including an active region registered with the cavitywith a portion of the piezoelectric material arranged betweenoverlapping portions of a top side electrode and a bottom sideelectrode.

FIGS. 11A and 11B are graphical representations of the c-axis angle ofsamples from an Example.

FIGS. 12A and 12B are graphical representations of the c-axis angle of acomparative sample from the Example.

FIG. 13 is a graphical representation of electromechanical coupling ofsamples and the comparative sample of the Example.

FIG. 14 is a graphical representation of wafer quality of samples andthe comparative sample of the Example.

DETAILED DESCRIPTION

The present disclosure relates to methods of depositing crystalline bulklayers that allow for selecting the c-axis tilt. In particular, thepresent disclosure relates to methods of depositing crystalline bulklayers that allow for selecting c-axis tilt angles that are greater thanangles achieved with prior art methods (e.g., greater than about 28 to30 degrees for AlN). The methods of the present disclosure may result inbulk layers where the crystals of the bulk layer are substantiallyoriented with one another.

The present disclosure also relates to methods of depositing crystallinebulk layers at an inclined c-axis directly on a substrate without a seedlayer.

One or both of achieving a desired c-axis tilt and directly depositing apiezoelectric bulk layer on a substrate (without an intervening seedlayer) may result in improved characteristics of the resultingstructure, such as improved coupling efficiency, improved mechanicalquality factor, and improved shear to longitudinal coupling ratios.

When referring to c-axis tilt or c-axis orientation, it should beunderstood that even if a single angular value is given, the crystals ina deposited crystal layer (e.g., a seed layer or a bulk layer) mayexhibit a distribution of angles. The distribution of angles typicallyapproximately follows a normal (e.g., Gaussian) distribution that can begraphically demonstrated, for example, as a two-dimensional plotresembling a bell-curve, or by a pole figure.

While some prior art methods (such as those described in U.S. patentapplication Ser. No. 15/293,063 entitled “Deposition System for Growthof Inclined C-Axis Piezoelectric Material Structures;” U.S. patentapplication Ser. No. 15/293,071 entitled “Methods for FabricatingAcoustic Structure with Inclined C-Axis Piezoelectric Bulk andCrystalline Seed Layers;” U.S. patent application Ser. No. 15/293,082entitled “Acoustic Resonator Structure with Inclined C-AxisPiezoelectric Bulk and Crystalline Seed Layers;” U.S. patent applicationSer. No. 15/293,091 entitled “Multi-Stage Deposition System for Growthof Inclined C-Axis Piezoelectric Material Structures;” and U.S. patentapplication Ser. No. 15/293,108 entitled “Methods for ProducingPiezoelectric Bulk and Crystalline Seed Layers of Different C-AxisOrientation Distributions”) are suitable for producing bulk acousticwave resonator structures with bulk layer crystals deposited over a seedlayer and having an inclined c-axis crystal structure and an improvedincline distribution, those methods are still limited by the typicalc-axis tilt of the material used for making the bulk layer, and requirethe deposition of a seed layer first for a successful deposition of abulk layer of desired quality. For example, bulk layers prepared fromAlN according to methods described in the above-mentioned disclosuresare limited to a c-axis tilt distribution that is centered around about27 degrees to about 30 degrees. The bulk layers may have a distributionwhere at least 75% (or at least 90%, or at least 95%) of the bulk layercrystals have a c-axis orientation between 25 degrees to 50 degreesrelative to the normal of the substrate, with the highest frequencyoccurring at about 27 to 30 degrees. A typical c-axis orientationdistribution resulting from a prior art method is demonstrated in FIGS.6B and 6C. In FIG. 6B, the bulk layer was deposited over a seed layerprepared at a pressure of 5 mTorr. In FIG. 6C, the bulk layer wasdeposited over a seed layer prepared at a pressure of 15 mTorr. As canbe seen, the highest frequency of c-axis orientation occurs at about27-28 degrees when the bulk layer is deposited on a seed layer. FIG. 6Adepicts the c-axis orientation distribution of a bulk layer depositedwithout a seed layer under prior art deposition conditions, whichresults in a c-axis tilt substantially less than when deposited on aseed layer (compare to FIGS. 6B and 6C). Due, at least in part, to thesubstantially lower c-axis tilt when no seed layer was employed, theshear to longitudinal coupling ratio was substantially less when no seedlayer was employed (ratio less than 0.5 when no seed layer vs. ratiogreater than 1.5 with seed layer deposited at 15 mTorr).

The desired c-axis tilt depends on the intended purpose, use, and effectof the bulk layer. For example, in some cases it may be desirable toincrease shear mode excitation. As discussed previously, the shearcoupling coefficient for a bulk acoustic wave resonator comprising abulk layer of AlN exceeds the longitudinal coupling coefficient forc-axis angle of inclination values in a range of from about 19 degreesto about 63 degrees. A greater difference between the shear mode andlongitudinal coupling is achieved approximately between 30 degrees and52 degrees, and a pure shear mode resonance response (with zerolongitudinal coupling) can be obtained at a c-axis angle of inclinationof about 46 degrees. Therefore, it would be desirable to be able toprepare an AlN bulk layer with a c-axis tilt of between about 30 degreesand about 52 degrees, between about 32 degrees and about 50 degrees,between about 35 degrees and about 48 degrees, or about 46 degrees. Insome embodiments, shear mode excitation may be increased by depositing abulk layer with a c-axis tilt of about 30 degrees to about 52 degrees,about 32 degrees to about 50 degrees, or about 35 degrees to about 48degrees. Other angles of the c-axis tilt may also be useful in otherembodiments. For example, c-axis tilts of about 30 degrees to about 45degrees, about 32 degrees, or about 90 degrees could be of interest insome embodiments.

The term “c-axis” is used here to refer to the (002) direction of adeposited crystal with a hexagonal wurtzite structure. The c-axis istypically the longitudinal axis of the crystal.

The terms “c-axis tilt,” “c-axis orientation,” and “c-axis incline” areused here interchangeably to refer to the angle of the c-axis relativeto a normal of the surface plane of the deposition substrate.

The term “incidence angle” is used here to refer to the angle at whichatoms are deposited onto a substrate, measured as the angle between thedeposition pathway and a normal of the surface plane of the substrate.

The term “substrate” is used here to refer to a material onto which aseed layer or a bulk layer may be deposited. The substrate may be, forexample, a wafer, or may be a part of a resonator device complex orwafer, which may also include other components, such as an electrodestructure arranged over at least a portion of the substrate. However, aseed layer is not considered to be “a substrate” in the embodiments ofthis disclosure.

The term “substantially” as used here has the same meaning as “nearlycompletely,” and can be understood to modify the term that follows by atleast about 90%, at least about 95%, or at least about 98%.

The terms “parallel” and “substantially parallel” with regard to thecrystals refer to the directionality of the crystals. Crystals that aresubstantially parallel not only have the same or similar c-axis tilt butalso point in the same or similar direction.

The term “about” is used here in conjunction with numeric values toinclude normal variations in measurements as expected by persons skilledin the art, and is understood have the same meaning as “approximately”and to cover a typical margin of error, such as +5% of the stated value.

All scientific and technical terms used herein have meanings commonlyused in the art unless otherwise specified. The definitions providedherein are to facilitate understanding of certain terms used frequentlyherein and are not meant to limit the scope of the present disclosure.

As used here, the singular forms “a”, “an”, and “the” encompassembodiments having plural referents, unless the content clearly dictatesotherwise.

As used here, the term “or” is generally employed in its sense including“and/or” unless the content clearly dictates otherwise. The term“and/or” means one or all of the listed elements or a combination of anytwo or more of the listed elements.

As used here, “have”, “having”, “include”, “including”, “comprise”,“comprising” or the like are used in their open-ended sense, andgenerally mean “including, but not limited to.” It will be understoodthat “consisting essentially of,” “consisting of,” and the like aresubsumed in “comprising” and the like. As used herein, “consistingessentially of,” as it relates to a composition, product, method or thelike, means that the components of the composition, product, method orthe like are limited to the enumerated components and any othercomponents that do not materially affect the basic and novelcharacteristic(s) of the composition, product, method or the like.

The words “preferred” and “preferably” refer to embodiments of theinvention that may afford certain benefits, under certain circumstances.However, other embodiments may also be preferred, under the same orother circumstances. Furthermore, the recitation of one or morepreferred embodiments does not imply that other embodiments are notuseful, and is not intended to exclude other embodiments from the scopeof the disclosure, including the claims.

The recitations of numerical ranges by endpoints include all numberssubsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, 5, etc. or 10 or less includes 10, 9.4, 7.6, 5, 4.3, 2.9, 1.62,0.3, etc.). Where a range of values is “up to” a particular value, thatvalue is included within the range.

Any direction referred to here, such as “top,” “bottom,” “left,”“right,” “upper,” “lower,” and other directions and orientations aredescribed herein for clarity in reference to the figures and are not tobe limiting of an actual device or system or use of the device orsystem. Devices or systems as described herein may be used in a numberof directions and orientations.

The present disclosure relates in various aspects to bulk acoustic waveresonator structures and to methods for fabricating such resonatorstructures. As compared to conventional resonator structures,fabrication methods, and deposition systems, various embodiments of thepresent disclosure include or enable inclined c-axis piezoelectric filmswith a preselected c-axis tilt angle with an increased range ofselectable angles. The inclined c-axis piezoelectric films may alsoexhibit improved mechanical quality factor, reduced acoustic losses,and/or reduced ohmic (electrical) losses. The inclined c-axispiezoelectric films may be fabricated over large areas (e.g., large areasubstrates) with increased uniformity of c-axis tilt angle. The methodsfor making the inclined c-axis piezoelectric films may be simpler and/orinclude fewer steps than prior art methods for preparing inclined c-axispiezoelectric films.

In various aspects, bulk acoustic wave resonator structures and methodsfor fabricating such resonator structures include deposition of the bulklayer directly on the substrate (without a seed layer). Improvedcoupling efficiency and mechanical quality factor may result from theelimination of a layer (e.g., a seed layer), even when the layer (e.g.,seed layer) might be formed from the same material as the bulk layer. Insome embodiments, the resonator structures are formed by depositing thebulk layer directly on the substrate, and exhibit an improved mechanicalquality factor, reduced acoustic losses, and/or reduced ohmic(electrical) losses. The resonator structures also exhibit increaseduniformity of the c-axis tilt angle over large areas.

According to at least some embodiments of the present disclosure, thec-axis tilt of the bulk layer may be adjusted by depositing the bulklayer at the desired angle under certain initial deposition conditions.According to some embodiments, the initial deposition conditions aresuch that they retard the surface mobility of atoms while the bulk layeris being deposited. In at least some embodiments, the bulk layer with apre-selected c-axis tilt can be deposited under the initial depositionconditions without first depositing a seed layer. Without wishing to bebound by theory, it is believed that slowing down the surface mobilityof atoms favors kinetics over thermodynamics, and allows the atoms beingdeposited to respond to changes in the deposition environment.

The bulk layers of the present disclosure can be prepared in anysuitable deposition system. One example of a suitable deposition systemis described in U.S. patent application Ser. No. 15/293,063 entitled“Deposition System for Growth of Inclined C-Axis Piezoelectric MaterialStructures.” The main aspects of the deposition system are summarizedbelow. However, the methods of the present disclosure are notparticularly limited by the system used, and other suitable systems mayalso be used.

The crystalline layers of the present disclosure can be prepared in adeposition system incorporating a multi-aperture collimator arrangedbetween a target surface of a linear sputtering apparatus and asubstrate table that supports one or more wafers or substrates forreceiving sputter-deposited material.

An exemplary deposition system is shown in FIG. 7, which is an upperexterior perspective view of the reactor 100 of the deposition systemfor growing a hexagonal crystal structure piezoelectric materials. Thereactor 100 includes first, second, and third tubular portions 102, 120,108 for housing various elements used for depositing material onto asubstrate. FIG. 8 depicts an upper perspective view of some of theelements of the reactor 100, including a linear sputtering apparatus154, a translation track 115 for translating a movable substrate tablefor supporting multiple substrates, and a collimator assembly 170.

The target surface may be non-parallel to the substrate table, and theintermediately arranged collimator may be non-parallel to both thetarget surface and the substrate table. The collimator and the substratetable are preferably both capable of movement (e.g., translation) duringsputtering, and at least one of the substrate table or the collimator ispreferably biased to an electrical potential other than ground. Thesystem may be used to grow (e.g., deposit) a crystalline seed layerduring a first step, followed by growth of a hexagonal crystal structurepiezoelectric material bulk layer over the crystalline seed layer duringa second step under conditions that differ from the first step.Alternatively, according to the methods of the present disclosure, thebulk layer may be deposited directly onto the substrate without firstdepositing a seed layer.

According to an embodiment, the bulk layer is grown (e.g., deposited) ina single step, using a single sputtering apparatus. The growth step maybe performed with a deposition system utilizing a linear sputteringapparatus, a substrate table that is translatable between differentpositions within the linear sputtering apparatus, and a collimatorarranged between the substrate table and the linear sputteringapparatus. A hexagonal crystal structure piezoelectric material bulklayer may be grown in an enclosure in which subatmospheric conditionsmay be generated using at least one vacuum pumping element, and a waferor substrate supporting the bulk layer may be translated within theenclosure. The linear sputtering apparatus, which may include a linearmagnetron or a linear ion beam sputtering apparatus, includes a targetsurface configured to eject metal (e.g., aluminum or zinc) atoms, withthe target surface being non-parallel to (e.g., oriented at 0 to lessthan 90 degrees from) the support surface. Preferably, the collimator isalso arranged non-parallel to the support surface. In certainembodiments, a target surface is arranged at a first nonzero anglerelative to a support surface, and a collimator is arranged at a secondnonzero angle relative to the support surface, wherein the first nonzeroangle is greater than the second nonzero angle. Metal atoms ejected fromthe target surface react with a gas species contained in agas-containing environment to form the material to be deposited (e.g.,piezoelectric material). For example, aluminum atoms ejected from analuminum or aluminum-containing target surface may react with nitrogengas species to form aluminum nitride, or zinc atoms ejected from a zincor zinc-containing target surface may react with oxygen gas species toform zinc oxide.

The support surface of the substrate table may be configured to receiveone or more wafers to be used as deposition substrates, preferablyhaving a diameter in a range of at least about 50 mm, about 100 mm, orabout 150 mm. The substrate table may be coupled to a movable element(e.g., a translation element) configured to move the substrate tableduring operation of the linear sputtering apparatus. Movement of thesubstrate table may promote uniform material deposition over large areasby preventing localized material deposition regions of differentthicknesses. The exemplary collimator includes multiple guide membersarranged non-parallel to the support surface, such as a plurality oflongitudinal members and a plurality of transverse members, that form agrid defining multiple collimator apertures. Electrical biasing of thesubstrate table and/or the collimator to a potential other than groundenhances control of material deposition during operating of thesputtering apparatus. Collimator biasing may also influencemicrostructure development of tilted c-axis piezoelectric bulk materialin a bulk acoustic wave resonator device. The substrate table and thecollimator may be independently biased to electrical potentials otherthan ground. Separate guide members of the collimator may also beelectrically biased differently relative to one another. The collimatormay be configured to translate during operation of the linear sputteringapparatus to prevent formation of a “shadow” pattern that couldotherwise be formed on a surface receiving deposited piezoelectricmaterial. A deposition aperture may be arranged between the collimatorand the substrate table.

According to some embodiments, an acoustic resonator structure isprepared in a deposition system where at least one wafer comprising asubstrate is received by a support surface. An acoustic reflectorstructure may be arranged over the substrate and an electrode structurearranged over at least a portion of the acoustic reflector structure, asmay be useful to produce at least one solidly mounted bulk acoustic waveresonator device. In certain other embodiments, the at least one waferincludes a substrate defining a recess, a support layer is arranged overthe recess, and an electrode structure is arranged over the supportlayer. In describing the method here, deposition simply onto “asubstrate” may be described. However, it is to be understood that thesubstrate may be a part of a resonator device complex or wafer, whichmay also include other components, such as an electrode structurearranged over at least a portion of the substrate.

According to at least some embodiments of the present disclosure, thebulk layer has a c-axis tilt that can be pre-selected. The methods ofthe present disclosure result in a bulk layer where the crystals of thebulk layer align with or substantially align with the pre-selectedc-axis tilt. In some embodiments, the distribution of the c-axis tilt ofthe crystals is such that at least 75%, at least 80%, at least 85%, atleast 90%, or at least 95% of the crystals in the bulk layer have ac-axis tilt that is within a range of the pre-selected c-axis tilt, suchas within 1 to 10 degrees, within 1 to 8 degrees, within 1 to 5 degrees,or within 1 to 3 degrees of the pre-selected c-axis tilt. The crystalsof the bulk layer may also be substantially parallel with each other.For example, at least 50%, at least 75%, or at least 90% of the crystalsmay have a direction that is within 0 degrees to 45 degrees, or within 0degrees to 20 degrees of the average crystal direction.

In one aspect, the disclosure relates to a method for fabricating atleast one acoustic resonator structure, wherein the growth step includesdeposition of a hexagonal crystal structure bulk layer on a substrate.The deposited material may be piezoelectric material. The growth stepincludes ejection of metal atoms from a target surface of a linearsputtering apparatus to react with a gas species and to be received bythe substrate.

In some embodiments, the bulk layer is deposited under depositionconditions (or initial deposition conditions), including an incidenceangle. The incidence angle may be an off-normal incidence. For example,the incidence angle may be greater than 10 degrees, greater than 27degrees, greater than 30 degrees, greater than 32 degrees, greater than33 degrees, greater than 34 degrees, greater than 35 degrees, greaterthan 36 degrees, or greater than 40 degrees. The incidence angle may beup to about 85 degrees, up to about 75 degrees, up to about 65 degrees,up to about 56 degrees, up to about 52 degrees, up to about 50 degrees,up to about 49 degrees, or up to about 48 degrees. Illustrativeincidence angles include 35 degrees, 40 degrees, 43 degrees, and 46degrees. In some embodiments, the incidence angle is less than 32degrees or greater than 40 degrees.

Under the deposition conditions, the resulting bulk material crystalsmay have a c-axis that aligns or at least substantially aligns with theincidence angle used during the deposition of the bulk layer. Theresulting bulk layer crystals are substantially parallel to one anotherand at least substantially align with the desired c-axis tilt.

Without wishing to be bound by theory, it is hypothesized that thedeposition conditions may be selected such that the depositionconditions retard the surface mobility of the atoms being deposited. Thedeposition conditions that may have a surface mobility retarding effectinclude many variables. These variables may be selected so that surfacemobility is decreased to the point that the c-axis tilt of the seedlayer and/or bulk layer can be controlled. The surface mobility of theatoms is a result of the variables as a whole, and not necessarily anysingle variable alone. When compared to conventional methods anddeposition conditions for depositing bulk layers, each of the variablesmay be somewhat different, or only some of the variables may bedifferent while others may remain the same as in conventional methods.Because surface mobility of atoms is difficult to determine directly,the combination of appropriate conditions may be determined based on theability to change the c-axis tilt of the resulting crystalline layerbeyond the angle(s) that is typically available for a deposited materialdue to crystallographic restrictions. For example, in the case of AlN,the ability to produce a crystalline layer with a c-axis tilt alignedalong 32 degrees (with distribution of angles ranging from about 25 toabout 35 degrees) or greater may indicate deposition conditions thatfavor kinetics over thermodynamics and that allow crystal growth torespond to changes in the deposition environment. Also, the ability todeposit a bulk layer directly onto a substrate (without a seed layer) ata c-axis tilt angle aligned along 20 degrees, above 25 degrees, above 30degrees, or above 35 degrees may indicate deposition conditions thatfavor kinetics over thermodynamics. The crystals in the bulk layer maybe aligned or substantially aligned over the area of the substrate(e.g., over the entire deposition area).

The bulk layer may be deposited directly onto a substrate (e.g., awafer) without first depositing a seed layer. According to anembodiment, the bulk layer may be deposited under the selecteddeposition conditions at an off-normal incidence angle such thatcrystals in the resulting initial bulk layer have the desired c-axistilt.

According to some embodiments, the deposition conditions include one ormore of pressure, temperature, distance from the target to thesubstrate, and gas ratio. The pressure may be at least about 0.5 mTorr,at least about 1 mTorr, or at least about 1.5 mTorr. The pressure may beup to about 10 mTorr, up to about 8 mTorr, or up to about 6 mTorr. Insome embodiments, the pressure is below 5 mTorr. For example, thepressure may be about 2 mTorr, about 2.5 mTorr, about 3 mTorr, about 3.5mTorr, or about 4 mTorr. The temperature may be at least about 20° C.,at least about 50° C., or at least about 100° C. The temperature may beup to about 300° C., up to about 250° C., or up to about 200° C. In someembodiments, the deposition process may generate heat but the depositionchamber is not heated by a heater.

The distance from the target to the substrate during deposition may beat least about 50 mm, at least about 75 mm, at least about 80 mm, or atleast about 90 mm. The distance may be up to about 200 mm, up to about150 mm, up to about 130 mm, or up to about 120 mm. In some embodiments,the distance from the target to the substrate during deposition may beabout 108 mm to about 115 mm.

The gases in the vapor space of the deposition system may be selectedbased on the intended composition of the deposited layer, and mayinclude argon and a gas that reacts with the deposited atoms, such asnitrogen or oxygen. The gas ratio of argon to reacting gas (e.g.,nitrogen) in the vapor space may be from about 1:10 to about 10:10, fromabout 2:10 to about 8:10, or about 4:10.

The preselected c-axis tilt angle will depend on the desired or intendeduse of the resulting crystalline bulk layer structure. For example, thepreselected angle may be any angle greater than 0 degrees and less than90 degrees. It may be desirable to select an angle that favors shearmode resonance. For example, the preselected angle may be greater than10 degrees, greater than 27 degrees, greater than 30 degrees, greaterthan 32 degrees, greater than 33 degrees, greater than 34 degrees,greater than 35 degrees, greater than 36 degrees, or greater than 40degrees. The preselected angle may be up to about 85 degrees, up toabout 75 degrees, up to about 65 degrees, up to about 56 degrees, up toabout 52 degrees, up to about 50 degrees, up to about 49 degrees, or upto about 48 degrees. Exemplary preselected angles include 35 degrees and46 degrees. In some embodiments, the preselected angle is less than 32degrees or greater than 46 degrees.

According to at least some embodiments, the c-axis tilt of the resultingbulk layer is the same as the preselected angle or is within a range ofthe incidence angle and/or the preselected angle. For example, thec-axis tilt of the resulting bulk layer may be within 1 degree, within 2degrees, within 3 degrees, within 5 degrees, within 10 degrees, orwithin 15 degrees of the incidence angle and/or the pre-selected c-axistilt. The distribution of the c-axis tilt of the bulk layer crystals maybe such that at least 75%, at least 80%, at least 85%, at least 90%, orat least 95% of the crystals in the bulk layer have a c-axis tilt thatis within a range, such as within 1 degree, within 2 degrees, within 3degrees, within 5 degrees, within 10 degrees, or within 15 degrees, ofthe incidence angle and/or the pre-selected c-axis tilt.

The surface of the substrate may optionally be roughened prior todeposition of the bulk layer. Roughening of the surface may improve theability of the subsequently grown bulk layer crystals to orient duringthe deposition. Without wishing to be bound by theory, it is believedthat roughening the surface causes shadowing effects, which may helpfavor orientation of the crystals toward the angle of deposition. Thesurface of the substrate may be roughened by, for example, atomicbombardment, creating “hills” and “valleys” on the surface.

Suitable materials for the bulk layer include piezoelectric materials orother metallic materials with a high melting point. In some embodimentsthe material includes a metal nitride, such as aluminum nitride,titanium nitride, hafnium nitride, tantalum nitride, zirconium nitride,vanadium nitride, niobium nitride, etc. In some embodiments the materialincludes a metal oxide, such as zinc oxide, tungsten oxide, hafniumoxide, molybdenum oxide, etc. In some embodiments, the materialcomprises a metal oxynitride, such as hafnium oxynitride, titaniumoxynitride, tantalum oxynitride, etc. In some embodiments the materialincludes a metal carbide such as titanium carbide, niobium carbide,tungsten carbide, tantalum carbide, etc. In some embodiments thematerial is a refractory metal, such as zirconium, hafnium, tungsten,molybdenum, etc. The bulk layer may comprise a combination of two ormore of the material described above.

In certain embodiments, a substrate table and/or collimator isconfigured to translate during the growth step to promote uniformmaterial deposition. An electrode structure may be formed over at leastone portion of the hexagonal crystal structure piezoelectric materialbulk layer to form at least one bulk acoustic wave resonator device. Anactive region of bulk acoustic wave resonator device is provided in anarea in which the hexagonal crystal structure piezoelectric materialbulk layer is arranged between a first electrode structure and a secondelectrode structure. Such growth steps may be performed using a singlesputtering apparatus or a deposition system utilizing multiple linearsputtering apparatuses, a substrate table that is translatable betweendifferent positions proximate to different linear sputteringapparatuses, and multiple collimators arranged between the substratetable and the respective linear sputtering apparatuses. In certainembodiments, at least one resonator device complex, over which thehexagonal crystal structure piezoelectric material bulk layer isdeposited, is diced into a plurality of chips, such as solidly mountedbulk acoustic wave resonator chips or film bulk acoustic wave resonatorchips.

The distribution of the c-axis orientation of the hexagonal crystalstructure piezoelectric material bulk layer may be normal or bimodal. Ina preferred embodiment, the distribution is normal. In certainembodiments, less than about 30%, less than about 25%, or less thanabout 20% of the c-axis orientation distribution of the hexagonalcrystal structure piezoelectric material bulk layer is in a range offrom 0 degrees to 25 degrees relative to normal of a face of thesubstrate. In certain embodiments, less than about 30%, less than about25%, or less than about 20% of the c-axis orientation distribution ofthe hexagonal crystal structure piezoelectric material bulk layer is ina range of from 45 degrees to 90 degrees relative to normal of a face ofthe substrate. At least 60%, at least 65%, at least 70%, at least 75%,at least 80%, at least 85%, or at least 90% of the c-axis orientationdistribution of the hexagonal crystal structure piezoelectric materialbulk layer may be in a range of 25 degrees to 45 degrees. In someembodiments, at least 60%, at least 65%, at least 70%, at least 75%, atleast 80%, at least 85%, or at least 90% of the c-axis orientationdistribution of the hexagonal crystal structure piezoelectric materialbulk layer is in a range of 30 degrees to 40 degrees.

In certain embodiments, the substrate comprises a diameter of at leastabout 50 mm (or at least about 100 mm, or at least about 150 mm) and thehexagonal crystal structure piezoelectric material bulk layer covers atleast about 50% (or at least about 75%, or at least about 90%, or atleast about 95%) of a face of the substrate. In certain embodiments,multiple bulk acoustic wave resonator devices each including an activeregion between a first electrode structure and a second electrodestructure are provided on a single substrate. Multiple bulk acousticresonator chips may be derived from such a substrate (e.g., by dicing),and may be incorporated in one or more sensors and/or fluidic devices.

In certain embodiments, the deposition system is configured for growthof a hexagonal crystal structure piezoelectric material bulk layerdirectly over a substrate (without first depositing a crystalline seedlayer). The substrate may be a wafer received by the support surface,wherein at least 50% (or at least 75%, or at least 90%, or at least 95%)of the hexagonal crystal structure piezoelectric material bulk layercomprises a c-axis having an orientation distribution predominantly in arange of from 25 degrees to 50 degrees (or in a subrange of from 30degrees to 40 degrees with a peak at about 35 degrees), or greater than27 degrees, greater than 30 degrees, greater than 32 degrees, greaterthan 33 degrees, greater than 34 degrees, greater than 35 degrees,greater than 36 degrees, or greater than 40 degrees, relative to normalof a face of a substrate or wafer received by the support surface. Theorientation distribution may be up to about 85 degrees, up to about 75degrees, up to about 65 degrees, up to about 56 degrees, up to about 52degrees, up to about 50 degrees, up to about 49 degrees, or up to about48 degrees. Such c-axis orientation distribution is preferablysubstantially uniform over the area of a large area substrate (e.g.,having a diameter in a range of at least about 50 mm, about 100 mm, orabout 150 mm), thereby enabling multiple chips having the same orsimilar acoustic wave propagation characteristics to be derived from asingle substrate.

In certain embodiments, a hexagonal crystal structure piezoelectricmaterial bulk layer comprises a c-axis having an orientationdistribution predominantly in a range of from 12 degrees to 52 degrees,or in a range of from 27 degrees to 37 degrees, or in a range of from 75degrees to 90 degrees, relative to normal of a face of a substrate orwafer supporting the hexagonal crystal structure piezoelectric materialbulk layer. In certain embodiments, the hexagonal crystal structurepiezoelectric material bulk layer may have a thickness of about 1,000 Å(Angstrom) or greater, about 2,000 Å or greater, about 3,000 Å orgreater, about 4,000 Å or greater, about 6,000 Å or greater, or about10,000 Å or greater. The thickness of the hexagonal crystal structurepiezoelectric material bulk layer may be up to about 30,000 Å, up toabout 26,000 Å, or up to about 20,000 Å. Such hexagonal crystalstructure piezoelectric material bulk layer preferably includessubstantially uniform thickness, nanostructure, and crystallinityproperties, with controlled stress and densely packed columnar grains orrecrystallized grain structure. In certain embodiments, a crystallineseed layer includes a thickness in a range of from 500 Å to 2,000 Å, and(for a hexagonal crystal structure piezoelectric material such as AlN)may include a dominant (103) texture. In other embodiments, thestructure does not include a seed layer.

The piezoelectric material films with a bulk layer according toembodiments of the present disclosure can be used in various bulkacoustic wave (“BAW”) devices, such as BAW resonators. Exemplary BAWresonators employing the piezoelectric material films of the presentdisclosure are shown in FIGS. 9-11.

FIG. 9 is a schematic cross-sectional view of a portion of a bulkacoustic wave solidly mounted resonator device 50 including apiezoelectric material bulk layer 64 embodying an inclined c-axishexagonal crystal structure piezoelectric material (e.g., AlN or ZnO) asdisclosed herein. The c-axis (or (002) direction) of the piezoelectricmaterial bulk layer 64 is tilted away from a direction normal to thesubstrate 52, as illustrated by two arrows superimposed over thepiezoelectric material bulk layer 64. The resonator device 50 includesthe substrate 52 (e.g., typically silicon or another semiconductormaterial), an acoustic reflector 54 arranged over the substrate 52, thepiezoelectric material bulk layer 64, and bottom and top side electrodes60, 68. The bottom side electrode 60 is arranged between the acousticreflector 54 and the piezoelectric material bulk layer 64, and the topside electrode 68 is arranged along a portion of an upper surface 66 ofthe piezoelectric material bulk layer 64. An area in which thepiezoelectric material bulk layer 64 is arranged between overlappingportions of the top side electrode 68 and the bottom side electrode 60is considered the active region 70 of the resonator device 50. Theacoustic reflector 54 serves to reflect acoustic waves and thereforereduce or avoid their dissipation in the substrate 52. In certainembodiments, the acoustic reflector 54 includes alternating thin layers56, 58 of materials of different acoustic impedances (e.g., SiOC, Si₃N₄,SiO₂, AlN, and Mo), optionally embodied in a Bragg mirror, depositedover the substrate 52. In certain embodiments, other types of acousticreflectors may be used. Steps for forming the resonator device 50 mayinclude depositing the acoustic reflector 54 over the substrate 52,followed by deposition of the bottom side electrode 60, followed bygrowth (e.g., via sputtering or other appropriate methods) of thepiezoelectric material bulk layer 64, followed by deposition of the topside electrode 68.

FIG. 10 is a schematic cross-sectional view of a film bulk acoustic waveresonator (FBAR) device 72 according to one embodiment. The FBAR device72 includes a substrate 74 (e.g., silicon or another semiconductormaterial) defining a cavity 76 that is covered by a support layer 78(e.g., silicon dioxide). A bottom side electrode 80 is arranged over aportion of the support layer 78, with the bottom side electrode 80 andthe support layer 78. A piezoelectric material bulk layer 84 embodyinginclined c-axis hexagonal crystal structure piezoelectric material(e.g., AlN or ZnO) is arranged over the bottom side electrode 80, and atop side electrode 88 is arranged over at least a portion of a topsurface 86 of the piezoelectric material bulk layer 84. A portion of thepiezoelectric material bulk layer 84 arranged between the top sideelectrode 88 and the bottom side electrode 80 embodies an active region90 of the FBAR device 72. The active region 90 is arranged over andregistered with the cavity 76 disposed below the support layer 78. Thecavity 76 serves to confine acoustic waves induced in the active region90 by preventing dissipation of acoustic energy into the substrate 74,since acoustic waves do not efficiently propagate across the cavity 76.In this respect, the cavity 76 provides an alternative to the acousticreflector 54 illustrated in FIG. 9. Although the cavity 76 shown in FIG.10 is bounded from below by a thinned portion of the substrate 74, inalternative embodiments at least a portion of the cavity 76 extendsthrough an entire thickness of the substrate 74. Steps for forming theFBAR device 72 may include defining the cavity 76 in the substrate 74,filling the cavity 76 with a sacrificial material (not shown) optionallyfollowed by planarization of the sacrificial material, depositing thesupport layer 78 over the substrate 74 and the sacrificial material,removing the sacrificial material (e.g., by flowing an etchant throughvertical openings defined in the substrate 74 or the support layer 78,or lateral edges of the substrate 74), depositing the bottom sideelectrode 80 over the support layer 78, growing (e.g., via sputtering orother appropriate methods) the piezoelectric material bulk layer 84, anddepositing the top side electrode 88.

In certain embodiments, an acoustic reflector structure is arrangedbetween the substrate and the at least one first electrode structure toprovide a solidly mounted bulk acoustic resonator device. Optionally, abackside of the substrate may include a roughened surface configured toreduce or eliminate backside acoustic reflection. In other embodiments,the substrate defines a recess, a support layer is arranged over therecess, and the support layer is arranged between the substrate and atleast a portion of the at least one first electrode structure, toprovide a film bulk acoustic wave resonator structure.

EXAMPLE

BAW wafers (samples) and a blanket film were prepared according tomethods of the present disclosure and compared to a baseline BAW wafer(comparative sample) and blanket film prepared according to prior artmethods.

All samples, including the comparative sample, were prepared using adeposition system as described in U.S. patent application Ser. No.15/293,063 entitled “Deposition System for Growth of Inclined C-AxisPiezoelectric Material Structures.”

Four sample wafers (150 mm diameter) and a blanket film were prepared bydepositing an AlN crystalline bulk layer directly onto a substratewithout prior application of a seed layer. The AlN crystalline bulklayer was deposited at a deposition angle of 43 degrees. The depositionpressure was selected at 2.5 mTorr. The power, target voltage and gasflow rate were manipulated to accommodate the pressure and to achieve adeposition rate of 40 Å/min.

A comparative (baseline) sample (150 mm diameter) and a comparativeblanket film were prepared by first depositing an AlN seed layer andthen depositing an AlN crystalline bulk layer onto the seed layer. Theseed layer was deposited at a deposition angle of 45 degrees. Thedeposition pressure was selected at 15 mTorr. The power, target voltageand gas flow rate were manipulated to accommodate the pressure and toachieve a deposition rate of 30 Å/min. The bulk layer was deposited at adeposition angle of 45 degrees. The deposition pressure was selected at1 mTorr. The power, target voltage and gas flow rate were manipulated toaccommodate the pressure and to achieve a deposition rate of 70 Å/min.

The blanket films were prepared to enable X-ray diffraction measurementof the c-axis angle of the AlN bulk-layer crystals. The AlN bulk-layercrystals deposited on the blanket films correspond to the AlN bulk-layercrystals deposited on the wafers under the same conditions.

The c-axis angle of the AlN bulk-layer crystals on the sample blanketfilm and the comparative blanket film was measured by a standard X-Raydiffractometer equipped with a goniometer for pole figure measurements.The results are graphically shown in FIGS. 11A and 11B for the sampleblanket film, and in FIGS. 12A and 12B for the comparative (baseline)blanket film.

It was observed that the samples had a c-axis tilt angle ofapproximately 35 degrees, whereas the comparative sample had a c-axistilt angle of approximately 30 degrees despite the deposition anglebeing 2 degrees greater in the comparative sample, as seen in the poleFIGS. 11A and 12A, respectively. It should be noted that the depositionangles given in this example are nominal settings of the depositionsystem and some variation may be experienced in the actual range ofangles at which the deposition flux contacts the substrate. However, therelative magnitude of the angles can still be compared.

The effective electromechanical coupling coefficient and mechanicalquality factor of each wafer were evaluated by investigating thescattering (S−) parameter matrices of the samples using a vector networkanalyzer to extract resonator performance characteristics. Electricalprobing was performed across 100 locations on each wafer, and theresults were calculated as normalized averages.

Methods for computing quality factor (Q) and effective couplingcoefficient (k² _(eff)) were based on work published by K. M. Lakin,“Modeling of Thin Film Resonators and Filters” IEEE MTT-S MicrowaveSymposium Digest, 1992 pp. 149-152. Quality factor is determinedutilizing the following expression:

$Q = {\frac{1}{2} \times {frequency} \times \frac{dZphase}{dfrequency}}$

Effective coupling coefficient is determined by measuring series (f_(s))and parallel (f_(p)) resonant frequencies and utilizing the followingformula:

$k_{eff}^{2} = {{\frac{\varphi\; s}{\tan\mspace{11mu}\varphi\; s}\mspace{14mu}{where}\mspace{14mu}\varphi\; s} = {\frac{\pi}{2}\left( \frac{fx}{fp} \right)}}$

The results are graphically shown in FIG. 13 showing electromechanicalcoupling coefficient (k_(e))² and in FIG. 13 showing mechanical qualityfactor (Q) normalized to the comparative (baseline) sample.

It was observed that the electrical performances of sample films grownunder deposition conditions according to the present disclosure werecomparable to or better than the comparative (baseline) sample. Theeffective electromechanical coupling coefficient (k_(e))² measured onthe sample wafers was approximately twice as good as the comparative(baseline) sample. The mechanical quality factor (Q) measured on thesample wafers was approximately 1.5 times better than the comparative(baseline) sample.

The sample c-axis angles achieved in this experiment were limited by theavailable incidence angles using the particular deposition system.However, the results demonstrate that angles can be selected outside ofthe 27-30 degrees of the prior art methods. Angles outside of 27-30degrees are achievable using the methods of the present disclosure.

The complete disclosures of the patents, patent documents, andpublications identified herein are incorporated by reference in theirentirety as if each were individually incorporated. To the extent thereis a conflict or discrepancy between this document and the disclosure inany such incorporated document, this document will control.

From the above disclosure of the general principles of the presentinvention and the preceding detailed description, those skilled in thisart will readily comprehend the various modifications, re-arrangementsand substitutions to which the present invention is susceptible, as wellas the various advantages and benefits the present invention mayprovide. Therefore, the scope of the invention should be limited only bythe following claims and equivalents thereof. In addition, it isunderstood to be within the scope of the present invention that thedisclosed and claimed articles and methods may be useful in applicationsother than surgical procedures. Therefore, the scope of the inventionmay be broadened to include the use of the claimed and disclosed methodsfor such other applications.

The invention claimed is:
 1. A method for preparing a crystalline bulklayer having a c-axis tilt, the method comprising: depositing a bulkmaterial layer directly onto a semiconductor substrate at an off-normalincidence of about 35 degrees to about 83 degrees under depositionconditions comprising a pressure of less than 4.5 mTorr, a temperatureof 50° C. to 300° C., a deposition distance between a target surface andthe semiconductor substrate of 50 mm to 200 mm, and gas ratio of argonto reacting gas of 1:10 to 10:10, wherein the depositing is performed ina deposition system comprising a linear sputtering apparatus, the targetsurface configured to eject metal atoms, a substrate table comprising asupport surface supporting the semiconductor substrate, and a collimatorarranged between the target surface and the semiconductor substrate, thetarget surface being at a first non-zero angle relative to thesemiconductor substrate and the collimator being arranged at a secondnon-zero angle relative to the support surface, wherein the secondnon-zero angle is different from the first non-zero angle, wherein thesemiconductor substrate and collimator are translated during deposition,and wherein the bulk material layer has a c-axis tilt of about 35degrees to about 85 degrees and exhibits a ratio of shear coupling tolongitudinal coupling of 1.25 or greater during excitation.
 2. Themethod of claim 1, wherein the pressure is from about 0.5 mTorr to 4.5mTorr.
 3. The method of claim 1, wherein the pressure is from about 1mTorr to about 4 mTorr.
 4. The method of claim 1, wherein the bulkmaterial layer has a thickness of about 1,000 Angstroms to about 30,000Angstroms.
 5. The method of claim 4, wherein the thickness is about3,000 Angstroms or greater.
 6. The method of claim 1, wherein thesubstrate table is biased to an electrical potential other than ground.7. The method of claim 1, wherein the collimator is biased to anelectrical potential other than ground.
 8. The method of claim 1,wherein the collimator comprises a grid formed by a plurality oflongitudinal members and a plurality of transverse members.
 9. Themethod of claim 8, wherein the plurality of longitudinal members of thecollimator are independently electrically biased relative to oneanother.
 10. A method for preparing a crystalline bulk layer having ac-axis tilt with a preselected angle, the method comprising: depositinga bulk material layer directly onto a semiconductor substrate at anoff-normal incidence under initial conditions that retard surfacemobility of the bulk material layer being deposited such that crystalsin the bulk material layer are substantially parallel to one another andare substantially oriented in a direction of the preselected angle,wherein the initial conditions comprise a pressure of less than 4.5mTorr, wherein the depositing is performed in a deposition systemcomprising a linear sputtering apparatus, a target surface configured toeject metal atoms, a substrate table comprising a support surfacesupporting the semiconductor substrate, and a collimator arrangedbetween the target surface and the semiconductor substrate, the targetsurface being at a first non-zero angle relative to the semiconductorsubstrate and the collimator being arranged at a second non-zero anglerelative to the support surface, wherein the second non-zero angle isdifferent from the first non-zero angle, and wherein the bulk materiallayer has a c-axis tilt of about 35 degrees to about 85 degrees andexhibits a ratio of shear coupling to longitudinal coupling of 1.25 orgreater during excitation.
 11. The method of claim 10, wherein theinitial conditions comprise a pressure of about 1 to 4.5 mTorr.
 12. Themethod of claim 10, wherein the initial conditions comprise an argon tonitrogen ratio in a vapor space of about 1:10 to about 10:10.
 13. Themethod of claim 10, wherein the preselected angle is about 35 degrees.14. The method of claim 10, wherein the preselected angle is about 46degrees.
 15. The method of claim 10, wherein the preselected angle isabout 50 degrees.
 16. The method of claim 10, wherein the bulk materiallayer comprises AlN.